The way I do it is put the tip of a pen in front of the image and focus on the image. That way you see two transparent copies of the pen. You'll see the image sort of repeats; move the pen forward and back between your face and the image until the two pen images line up with the repeats. Then focus on the pen and the image itself will divide into two in your vision. What I then find is that my brain is confused for a while, then suddenly the image will snap into focus and I can see the embedded image; it appears at a different "distance".

Looks like my subtle pun "sounds FISHY" was missed... I saw the first image and Saswat's COOL.